Why would you ever want to turn bluetooth off?
2) Since I imagine you'd like an example anyway - because I want to prevent my device from automatically connecting to headphones/speakers/cars that it has been paired to in the past.
BTW the Bluetooth button USED to turn BT off (same for WiFi) but it was changed to "do not connect to all visible BT devices for one hour" since so many people forgot to re enable their BT / WiFi and complained. So for you and me it is unwanted behaviour, for plenty of others it is now more what they expect.
Took me all of 10 seconds to set it up, thank you.
My phone never connects to pre-paired devices when in low-power mode (Apple-off) though.
In fact, I use it to force it to disconnect from devices - sometimes I do that to disconnect the phone from the headphones so I can connect them to another device without powering them off.
BLE is mostly for beacons, to find and suggest hotspots, airdrop, etc.
Power consumption is extremely low – rather than trying to micromanage that, you’ll see far more savings from using low-power mode earlier or uninstall the top consuming apps.